1. Field of the Invention
The present invention relates to a solid-state imaging device, a driving method and an electronic apparatus. More specifically, the present invention relates to a solid-state imaging device capable of reducing coupling noises, a method for driving the solid-state imaging device and an electronic apparatus employing the solid-state imaging device.
2. Description of the Related Art
As is generally known, a solid-state imaging device in related art employs a plurality of unit pixels each having a photodiode, a floating diffusion area and a transfer gate.
In every unit pixel employed in such a solid-state imaging device, in an operation to take an image of an imaging object, light generated by the imaging object is received by the photodiode 11 as shown in FIG. 1. The photodiode 11 converts the light into electric charge and accumulates the electric charge internally. Then, when a transfer pulse (or a transfer voltage) TRG is applied to the transfer-gate transistor 12 connected to the photodiode 11, the electric charge accumulated in the photodiode 11 is transferred to the floating diffusion area 13 by way of the transfer-gate transistor 12. The floating diffusion area 13 converts the electric charge into a voltage.
Then, when a select pulse (or a select voltage) SEL is applied to a select transistor 15 connected to a vertical signal line 14, the voltage generated by the floating diffusion area 13 is output to an external destination through an amplification transistor 16, the select transistor 15 and the vertical signal line 14 as a signal level.
In a state of applying the select pulse SEL to the select transistor 15 as it is, when a reset pulse (or a reset voltage) RST is applied to a reset transistor 17 later on, the voltage generated by the floating diffusion area 13 is reset to a voltage Vr determined in advance. Subsequently, a post-reset voltage generated by the floating diffusion area 13 is output to the external destination through the amplification transistor 16, the select transistor 15 and the vertical signal line 14 as a reset level.
The external destination computes the difference between the signal and reset levels output by the unit pixel as described above and takes the difference as the pixel signal of the unit pixel. It is worth noting that, in this specification, the technical term ‘pixel signal’ is used to imply the signal and/or reset levels themselves in some cases. It is to be noted that, if it is not necessary to hold electric charge in the floating diffusion area 13, the reset level can be read out in advance before the signal level is read out.
In general, in an operation to read out a pixel signal from a unit pixel in the solid-state imaging device, in order to eliminate noises such as threshold-voltage variations of the amplification transistor 16, the difference between the signal and reset levels is taken as the pixel signal in the so-called CDS (Correlated Double Sampling) processing.
At that time, since the signal and reset levels are read out from the unit pixel by making use of the same reset transistor 17, the same amplification transistor 16, the same select transistor 15 and the same vertical signal line 14, it is possible to eliminate fixed pattern noises such as threshold-voltage variations of the transistors.
By the way, a solid-state imaging device may have a function referred to as a global shutter function or a global exposure function. For more information on such a solid-state imaging device, the reader is advised to refer to documents such as Japanese Patent Laid-open Nos. 2001-238132 and 2009-268083. In such a solid-state imaging device, a global exposure operation is carried out on all unit pixels at the same time. In the global exposure operation, electric charge is transferred from the photodiode to the floating diffusion area for all unit pixels at the same time. With the electric charge held in the floating diffusion area, pixel signals are read out from the unit pixels sequentially.
Before the pixel signal is read out from the unit pixel, the floating diffusion area has been once reset in a batch operation carried out by a solid-state imaging device having the global shutter function to transfer electric charge from the photodiode to the floating diffusion area for all unit pixels at the same time or at the start of an exposure operation. In this case, electric charge will have been accumulated in the floating diffusion area at the time the pixel signal is read out from the unit pixel. Thus, in order to eliminate fixed pattern noises such as threshold-voltage variations of the amplification transistor 16, it is necessary to read out a reset level by resetting the floating diffusion area to a predetermined voltage after reading out the signal level.
For example, let pixel signals be read out sequentially from unit pixels provided on the (i-1)th pixel row and unit pixels provided on the ith pixel row on the light receiving surface of the solid-state imaging device as shown in FIG. 2.
In FIG. 2, the horizontal axis represents the lapse of time. Reference symbol SELi-1 denotes the voltage of the select pulse SEL applied to the gate electrode of the select transistor 15 employed in the unit pixel provided on the (i-1)th pixel row. On the other hand, reference symbol RSTi-1 denotes the voltage of the reset pulse RST applied to the gate electrode of the reset transistor 17 employed in the unit pixel provided on the (i-1)th pixel row. In addition, reference symbol TRGi-1 denotes the voltage of the transfer pulse TRG applied to the gate electrode of the transfer-gate transistor 12 employed in the unit pixel provided on the (i-1)th pixel row whereas reference symbol FDi-1 denotes the voltage appearing at the floating diffusion area included in the unit pixel provided on the (i-1)th pixel row.
By the same token, reference symbol SELi denotes the voltage of the select pulse SEL applied to the gate electrode of the select transistor 15 employed in the unit pixel provided on the ith pixel row. On the other hand, reference symbol RSTi denotes the voltage of the reset pulse RST applied to the gate electrode of the reset transistor 17 employed in the unit pixel provided on the ith pixel row. In addition, reference symbol TRGi denotes the voltage of the transfer pulse TRG applied to the gate electrode of the transfer-gate transistor 12 employed in the unit pixel provided on the ith pixel row whereas reference symbol FDi denotes the voltage appearing at the floating diffusion area included in the unit pixel provided on the ith pixel row.
In addition, reference symbol Vout denotes a voltage appearing on the vertical signal line 14 connected to unit pixels provided on the (i-1)th and ith pixel rows. To put it in detail, reference symbol Vout denotes a voltage appearing on the vertical signal line 14 connected to the select transistors 15 employed in unit pixels provided on the (i-1)th and ith pixel rows. The vertical signal line 14 is provided for every pixel column.
First of all, during a period TM11, an all-pixel simultaneous electronic shutter operation is carried out in the solid-state imaging device. The exposure operation is started for all unit pixels at the same time.
During the period TM11, on the (i-1)th pixel row, the voltage TRGi-1 of the transfer pulse TRG is raised in order to transfer electric charge from the photodiode to the floating diffusion area whereas the voltage RSTi-1 of the reset pulse RST is raised in order to reset the electric charge held in the floating diffusion area. By the same token, on the ith pixel row, the voltage TRG of the transfer pulse TRG is raised in order to transfer electric charge from the photodiode to the floating diffusion area whereas the voltage RSTi of the reset pulse RST is raised in order to reset the electric charge held in the floating diffusion area. At that time, the pixel signal is not read out from the unit pixel. Thus, the voltage SELi-1 and the voltage SELi of the select pulses SEL in all unit pixels are sustained at a low level.
It is to be noted that, in the following description, the operation to raise the voltage RSTi-1 or RSTi of the reset pulse RST applied to the gate electrode of the reset transistor 17 to a high level is referred to as an operation to activate the reset pulse RST (or activate the reset transistor 17). By the same token, the operation to raise the voltage TRGi-1 or TRGi of the transfer pulse TRG applied to the gate electrode of the transfer-gate transistor 12 to a high level is referred to as an operation to activate the transfer pulse TRG (or activate the transfer-gate transistor 12).
The all-pixel simultaneous electronic shutter operation is followed by a period TM12 during which an exposure operation is carried out at the same time on all unit pixels. After a period determined in advance has lapsed since the start of the period TM12, the reset pulse RST is activated for the unit pixels on the (i-1)th and ith pixel rows in order to reset the floating diffusion area. Then, later on, the transfer pulse TRG is activated for all unit pixels in order to transfer electric charge from the photodiode to the floating diffusion area in each of the unit pixels.
For example, in the unit pixel provided on the (i-1)th pixel row, the reset pulse RST is activated in order to reset the voltage FDi-1 appearing at the floating diffusion area to a voltage VFD1i-1′. After that, the transfer pulse TRG is activated in order to transfer electric charge from the photodiode to the floating diffusion area so that a voltage VFD1i-1 appears at the floating diffusion area.
The period TM12 is followed by the period TM13 during which pixel signals are read out sequentially from unit pixels. That is to say, during the period TM13, signal and reset levels are read out from the unit pixels.
First of all, a voltage is applied to the select transistor 15 of the unit pixel provided on the (i-1)th pixel row. That is to say, the voltage SELi-1 is raised from a low level to a high level in order to read out the voltage FDi-1 (=VFD1i-1) appearing at the floating diffusion area of the unit pixel provided on the (i-1)th pixel row. Thus, the voltage Vout appearing on the vertical signal line 14 is set at a voltage Vsig—i-1. In a period RD11, the voltage Vsig—i-1 is read out as the signal level of the unit pixel provided on the (i-1)th pixel row.
In addition, after the period RD11 has been ended, the reset pulse RST of the unit pixel provided on the (i-1)th pixel row is activated in order to reset the voltage FDi-1 appearing at the floating diffusion area of the unit pixel provided on the (i-1)th pixel row to a signal level VFD2i-1. Thus, the voltage Vout of the vertical signal line 14 is raised to a voltage Vrst—i-1. In a period RD12, the voltage Vrst—i-1 is read out as the reset level of the unit pixel provided on the (i-1)th pixel row. The difference between the signal and reset levels read out as described above is then output as a pixel signal.
After the operation to read out the pixel signal from the unit pixel provided on the (i-1)th pixel row has been completed, the voltage SELi-1 applied to the select transistor 15 is changed from the high level back to the low level. Then, an operation to read out the pixel signal from the unit pixel provided on the ith pixel row is carried out as follows.
A voltage is applied to the select transistor 15 employed in the unit pixel provided on the ith pixel row in order to read out the voltage FDi (=VFD1i) appearing at the floating diffusion area of the unit pixel provided on the ith pixel row. Thus, by applying the voltage to the select transistor 15, the voltage Vout of the vertical signal line 14 is set at a voltage Vsig—i. In a period RD13, the voltage Vsig—i is read out as the signal level of the unit pixel provided on the ith pixel row.
Then, after the period RD13 has been ended, the reset pulse RST of the unit pixel provided on the ith pixel row is activated in order to reset the voltage FDi appearing at the floating diffusion area of the unit pixel provided on the (i-1)th pixel row to a signal level VFD2i. Thus, the voltage Vout of the vertical signal line 14 is raised to a voltage Vrst—i. In a period RD14, the voltage Vrst—i is read out as the reset level of the unit pixel provided on the ith pixel row.
In the operation to obtain a pixel signal as described above, the signal and reset levels are read out through the same path to be used for computing a difference between the signal and reset levels. Thus, it is possible to eliminate fixed pattern noises such as threshold-voltage variations of the amplification transistor 16.